Abstract

The Kobbefjord Fault Zone (KFZ) is recognized as the largest of a group of early Proterozoic dextral transcurrent fault zones that cut Archaean gneisses in the vicinity of Godthåb. Detailed fjeld study of the KFZ reveals that it corresponds to a zone of deformation and alteration - 100 m wide, which completely obliterate pre-existing structures in country rock gneisses. Rocks within the KFZ are dominantly quartz-rich laminated mylonites characterized by regionally consistent NE-SW trending subvertical foliations and subhorizontallineations. Such mylonites were produced by intense ductile deformation, although local pseudotachylites and cataclasites occur which indicate minor amounts of brittIe deformation as well. Alteration in the KFZ involved pervasive pink staining and/or bleaching, and development of hydrated mineral assemblages (Chl + Ep + Musc), indicating an important role for fluid flux during the faulting process. Movement on the KFZ was probably initiated in the time period 2000-1800 Ma, as it displaces mafic dikes which appear correlative to dikes to the north of the Godthåb area which yield ages in this range. Deformation in the KFZ most plausibly represents internal deformation of a stable continental block subjected to compression on at least two sides. Data for fabric elements in the KFZ and for associated conjugate shears indicate that the principal compressive stress (o1) for this faulting episode was oriented W-NW, but precise relationships to deformation events on the margin of this block have yet to be deduced.

Highlights

  • The Kobbefjord Fault Zone (KFZ) is recognized as the largest of a group of early Proterozoic dextral transcurrent fault zones that cut Archaean gneisses in the vicinity of Godthåb

  • We describe and interpret the principal characteristics of relatively shallow level post-Archaean intracratonic fault zones in the region of Godthåbsfjord, West Greenland, based primarily on a detailed study of the Kobbefjord Fault Zone (KFZ), the largest and most continuous fault zone recognized in the area

  • The sense of displacement of paragneiss units on Storø Malene, the orientations of foliations and lineations, the slight angle between the fault trace and foliation strike, and the sense of rotation for mafic dikes deformed in the fault on Storø, all indicate dextral transcurrent movement for the KFZ

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Summary

Megascopic characteristics

The KFZ trends 040-0600 over a distance of > 80 km, - 35 of which is exposed on land (fig. 1). The fault traverses a quartzofeldspathic gneiss terrain along its entire length, and locally cuts anorthosite, Malene supracrustal rocks, and Q6rqut granite. The KFZ is subvertical along its entire length, but there are slight variations in the orientations of fault-related foliations and lineations from locality to 10caIity. Rccryslallized mylonile from zone af intense aiter.uian vicwed in plane- and cross-polarized light; blocky dark mineral is epidotc; c10udcd matcrial is feldspar (GRD-223, KFZ-Storø). The fault is developed sporadieally, and ean be reeognized by a characteristic salmon-pink to green staining and/ar bleaching. Such alteration represents retrograde hydration, carbonation and oxidation which occurrcd penecontemporaneously wilh fauJting

Fabric elements
MAP LEGEND
Matie dikes
AMITS UARSSUK
Duetile deformation
BritIle deformalion
Other faults iu the Godthåbsfjord region
Deformation history inferred for the KFZ
Deformation mechanism inferred for the KFZ
Age of faulting
Tectonic significance
Full Text
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